In general, carbon nanotube is widely used in various fields where carbonaceous nanomaterials are used. Carbon nanotube is an extremely fine cylindrical material with a very small diameter of several nanometers (nm). In the carbon nanotube, each carbon atom is bonded to three others carbon atoms, forming a hexagonal honeycomb structure. The carbon nanotube can be conducting or semiconducting depending on its structure and is expected to be widely applicable in various technical fields.
It is to utilize the advantages of the carbon nanotube over other existing materials, including high electrical conductivity and mechanical strength, fast redox reaction, excellent electron-emitting effect, superior cost competitiveness, or the like.
For growth of the carbonaceous nanomaterials including the carbon nanotube, various methods are reported, including arc discharge, laser vapor deposition, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition, or the like.
In arc discharge, graphite rods with different diameters are disposed in a vacuum chamber as an anode and a cathode to be spaced from each other and electrical discharge is induced. Carbon nanotube is formed on the outer surface of the chamber at the anode side. However, this method is not suitable for mass production and large amounts of impurities such as amorphous carbon or metal powder are formed. Thus, an additional purification process is necessary and the control of thickness and length of the carbon nanotube is not easy.
Laser vapor deposition synthesizes carbon nanotube via evaporation by irradiating laser to a graphite rod. Like the arc discharge method, this method is not suitable for mass production because the yield is extremely low.
In chemical vapor deposition, carbon nanotube is grown while flowing a carbon-containing precursor material into a high-temperature furnace. Although being advantageous for mass production, the method requires use of a catalyst and a high-temperature heat of 600-1000° C. Thus, a lot of efforts are required to remove the catalyst and glass or plastic substrates are inapplicable to the high-temperature process.
In plasma-enhanced chemical vapor deposition, a catalytic metal film is formed on a substrate and the catalytic metal film is etched using a plasma generated from an etching gas to form plural catalytic particles. Then, carbon nanotube is synthesized on the catalytic particles while supplying a carbon source gas to the plural catalytic particles formed on the substrate.
In the field emission display used for displaying and medical imaging, there has been an attempt to form carbon nanotube with high current density per unit area on a catalytic metal film formed on a semiconductor substrate to achieve high electron-emitting effect.
To utilize the grown carbonaceous nanomaterial, it needs to be attached to or deposited on the substrate of a device. The attachment may be achieved via direct growth on the substrate, for example, by chemical vapor deposition or via direct growth on another substrate and then transfer to the desired substrate. The adhesivity of the carbonaceous nanomaterial (e.g. graphene, carbon nanotube, carbon nanofiber, etc.) with the substrate depends on the van der Waals force which is in proportion to the area of contact. Since the existing carbon nanotube and carbon nanofiber have small diameters of several to hundreds of nanometers, the adhesivity to the substrate is very weak. As a consequence, operation time is short and degradation occurs easily.
To overcome this problem, a process of removing the grown material from the substrate, preparing it into a slurry and then attaching on a device substrate may be utilized. To the slurry, a binder is added to improve adhesion with the substrate. The slurry is changed into a thin film after being attached to the substrate following drying and baking. However, the thin film resulting from the attachment process has very poor electrical conductivity and other electrical properties as compared to direct growth owing to residual carbon resulting from the insulating binder material.
When carbon nanotube is used, electrical properties are degraded rapidly because of random horizontal arrangement during the preparation of the slurry in addition to the binder problem. Due to these disadvantages, desired efficiency, brightness, or the like expected from the theoretical calculation cannot be achieved. In addition, the carbonaceous nanomaterial is problematic in that device lifespan is reduced because of the heat generated during operation of the device.